As a healer, i prefer rdps to mdps only because of the extra damage i have to heal when we bring a mdps to a fight. On Zorn and Toth for exemple, if u have melee(s) on Toth, they take his aoe damage and if they manage to move out of it, we lose dps while they move. Or the first boss in LI HM, the tank has to move a lot more when mdps are present. Also, rdps can usually swap target faster then mdps. There is plenty of other exemple where i would prefer a rdps to a mdps.
But in the end, ive done EC HM with a few mdps, ive done LI HM with 2 mdps... it is no doubt still possible, but in my opinion having mdps does makes some fights harder. I dont think anyone can argue with that.
I am not a pro dps, since i mostly tank or heal, but i think it take more skill to be a good mdps vs being a good rdps (high dmg output with low dmg received).